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SubjectRe: [RFC][PATCH 2/3] sched, workqueue: Move WQ-sleeper wakeup
On Wed, 22 Jun 2011, Tejun Heo wrote:
> On Wed, Jun 22, 2011 at 01:34:46AM +0200, Peter Zijlstra wrote:
>
> Preemption could still be enabled here, right? What prevents
> preemtion kicking after wq_worker_sleeping() and do it again thus
> breaking nr_running tracking.

That wq code should serialize itself and not magically abuse rq->lock
for this.

> > * If we are going to sleep and we have plugged IO
> > * queued, make sure to submit it to avoid deadlocks.
> > */
> > @@ -4256,19 +4235,6 @@ asmlinkage void __sched schedule(void)
> > } else {
> > deactivate_task(rq, prev, DEQUEUE_SLEEP);
> > prev->on_rq = 0;
> > -
> > - /*
> > - * If a worker went to sleep, notify and ask workqueue
> > - * whether it wants to wake up a task to maintain
> > - * concurrency.
> > - */
> > - if (prev->flags & PF_WQ_WORKER) {
> > - struct task_struct *to_wakeup;
> > -
> > - to_wakeup = wq_worker_sleeping(prev, cpu);
> > - if (to_wakeup)
> > - try_to_wake_up_local(to_wakeup);
> > - }
>
> Similarly, the if the 'if {}' part of the above if/else is taken, the
> task never goes to sleep and nr_running will again be broken.

The accounting can be done at schedule entry and exit and not
somewhere magic in the wakeup code. It does really not matter, whether
nr_running is updated on wakeup or when the worker gets on the cpu.

Thanks,

tglx


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